Wireless communication networks include a number of Radio Access Nodes or RANs that provide access to a communication core network and a number of mobile stations or terminals. RANs are also commonly called base stations. In the 3GPP standardization of Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access Networks (E-UTRAN), a RAN is referred to as an evolved NodeB or eNodeB, while the mobile station or mobile terminal is referred to as User Equipment, UE. In LTE networks the access scheme for downlink communication, i.e. communication from the RAN to the UE, is Orthogonal Frequency Division Multiple Access (OFDMA), while the access scheme for uplink communication, i.e. communication from the UE to the RAN, is Single Carrier OFDMA (SC-OFDMA). UEs are time and frequency multiplexed on a physical uplink shared channel (PUSCH), which requires time and frequency synchronisation between an UE and the RAN.
A fundamental procedure in any cellular system is the random access procedure, which enables a mobile terminal or station to contact the network in an unsynchronised manner. In LTE wireless networks a non-synchronised uplink Random Access Channel (RACH) is used by the UE to send random access requests to the RAN. In response, the RAN sends back timing advance information to allow the UE to adjust its time alignment and thus to transmit successfully on the PUSCH. The random access procedure is used to request an initial access, to re-establish uplink (UL) synchronisation or as part of handover. As defined in 3GPP Technical Specification 36.300 the LTE random access procedure comes in two forms, allowing access to be either contention-based or contention-free. The contention-free random access procedure is used only to re-establish synchronisation prior to downlink data transmission and for incoming handover, when the UE contacts the RAN in the cell targeted for handover. The contention-based random access procedure may also be used for re-establishing synchronisation prior to downlink data transmission and for incoming handover, but it is also used for establishing initial access of the UE when it is in an idle state (i.e. the Radio Resource Control state: RRC-IDLE) and for re-establishing synchronisation prior to uplink data transmission. In both contention-based and contention-free random access procedures, the UE transmits a random access preamble to the RAN on the uplink RACH. In the contention-based random access procedure, the preamble is randomly chosen by the UE from a number of available preambles, with the result that more than one UE may transmit the same preamble simultaneously. Hence there is a need for signalling to resolve any contention. In the contention-free random access procedure, on the other hand, the RAN allocates a dedicated preamble to a UE, enabling contention-free access. This results in a faster procedure, which is particularly important for handover.
The UE initiates a contention-free random access (CFRA) procedure by transmitting the CFRA preamble. The RAN acknowledges receipt of the detected CFRA preamble by transmitting a random access response. This response includes a timing advance (TA) update to enable uplink synchronisation. The UE then adjusts the terminal transmit timing or time alignment using the TA update before transmitting a scheduled message on the uplink shared channel. This third message serves as an acknowledgment to the RAN that the random access response was received.
A problem may occur if the TA update information received by the UE is incorrect, as the UE will then not be able to synchronize with the network and not be able to transmit messages over the uplink shared channel (PUSCH) successfully. This may occur, for example, if the RAN fails to correctly detect the random access preamble sent by the UE, or rather, performs an estimate of transmission timing using different received preamble sequence yet assigns this to the UE so that an incorrect TA update is sent to the UE. This may occur in the case of very high speed UEs. Preambles are generated by cyclic shifts of a number of root sequences, which are configurable on a cell basis. At high velocity, frequency offset due to the Doppler shift causes spurious or aliased peaks. Sometimes a second peak or third peak of a given preamble sequence may shift into the detection window of an adjacent preamble and, depending on the power settings of the UE, with a signal strength that is greater than the detection threshold for the adjacent preambles. If the adjacent preamble is a contention-free random access preamble, the RAN may use the detected second or third peak to calculate time alignment then erroneously assign this TA to the adjacent preamble. Because these spurious peaks occur at known cyclic shift values, it is possible to reduce the risk of detecting an incorrect preamble by not using some cyclic shifts. The CFRA preamble set is then referred to as a restricted set. However even if UE is configured with such a restricted set, when the UE velocity is around 350 km/hour with a frequency offset in the range of around −1705 Hz to 1705 Hz, the third peak of an earlier preamble may have a sufficiently high signal strength to be detected in place of the real preamble. When the restricted set is not configured in the UE, frequency offsets greater than 625 Hz already allow both a second and a third peak of a preamble to become dominant in adjacent detection windows resulting in a higher risk of failure.
When the TA information used by the UE is incorrect, the subsequent uplink message sent to the RAN will fail. The RAN is expecting the uplink message, as this serves as an acknowledgment that the random access response and the information contained therein has been received. In the absence of this uplink message, or rather upon failure to detect this uplink message, the RAN thus sends a non acknowledgment to the UE by way of an automatic repeat request. The UE will then retransmit the message on the shared uplink channel (PUSCH). This process continues until the UE has retransmitted the message a maximum number of times, at which point no further non-acknowledgement messages are sent by the RAN. The UE takes no further action as it assumes that the CFRA procedure is successful after receipt of the random access response and may ultimately return to an idle mode. The delay caused by this failure is a particular problem when the CFRA procedure is used for handover as it may lead to call drops and negatively impact an operator's key performance indicators.
In the light of view of the problems associated with the prior art there is a need for an improved contention-free random access procedure.